Digital radiography is achieving a growing acceptance as an alternative to photographic-based imaging technologies that rely on photographic film layers to capture radiation exposure to produce and store an image of a subject's internal physical features. With digital radiography, the radiation image exposures captured on radiation sensitive layers are converted, pixel by pixel, to electronic image data which is then stored in memory banks for subsequent read-out and display on suitable electronic image display devices. One of the driving forces in the success of digital radiography is the ability to rapidly visualize and communicate stored images via data networks to one or more remote locations for analysis and diagnosis by radiologists without the delay caused by having to send physical films through the mail or via couriers to reach the remotely located radiologists.
Digital radiography panels have two-dimensional array of detecting elements (“pixels”) organized in rows and columns. To read out image information from the panel, rows of pixels are usually selected sequentially and the corresponding pixel on each column is connected to a charge amplifier. The outputs of the charge amplifiers from each column are applied to analog-to-digital converters to generate digitized image data that then can be stored and suitably image processed as needed for subsequent display.
In order to synchronize image acquisition and subsequent data readout from the imaging panel, it is necessary to synchronize control of the panel operation with the occurrence of impinging imaging X-rays from a remote X-ray source contained in the digital radiography imaging system. This can be done by communicating control signals indicating start and stop of the X-ray source via a cable wire tether. More recently, wireless imaging cassettes have been proposed that operate independently of the main system by using X-ray sensors in the imaging cassette to detect the onset and terminate of the impinging X-rays from the remote X-ray source. Examples of such wireless and/or independent X-ray impingement sensing are found in U.S. Pat. No. 6,069,935 (Schick). In one such example, dedicated X-ray event trigger diodes located in the imaging cassette outside the imaging panel are monitored by a computer to detect incident radiation and output a signal indicating same. Such a system has certain drawbacks. The inclusion of the trigger diodes lowers manufacturing yields thereby making the cassettes unduly costly. Also, the diodes themselves may be blocked by some radiation impervious portion of the object under test or may be out of the field of the radiation beam entirely. In another example described in this patent, the sensors of the imaging panel itself are continuously read out using frame-grabbing techniques. Determination of whether the imaging sensors were exposed to X-rays is made by continuously reading out the frames of data from the entirety of the panel sensors and determining whether the panel was exposed to X-rays by examining the frames of data. A drawback is that the sensors must be read out continuously which consumes a relatively high amount of electrical power which can be a serious problem for a battery-power cassette operating independently of the main imaging system.
Another example is found in U.S. Pat. No. 6,404,845 (Sharpless) in which certain reference pixels in the imaging panel are monitored during a wait for exposure period, with the values of the reference pixels being compared to a predetermined threshold level. When a predetermined number of the reference pixels exceed the threshold level, a determination is made that the exposure level has commenced. This approach, however, also consumes a high amount of power and is, therefore, a less than desirable solution.
Yet another example is found in US Published Application No. 2004/0065836 (Schick). In this example, the occurrence of X-radiation on an imaging panel is detected by monitoring the amount of current drawn by the imaging pixels in the panel and an X-ray occurrence signal is generated when the amount of current drawn exceeds a predetermined amount. However, the example disclosed in this application is limited to use with CMOS or CCD sensors and is not applicable to other types of sensors such as amorphous or crystalline silicon photodiodes or metal insulated semiconductor (MIS) sensors in extensive use in filmless imaging X-ray sensor panels.
In such latter systems, it is important to be able to detect, in a reliable manner, not only the onset of X-ray exposure but also its cessation to properly initiate timing and control operations associated with readout of the charge voltages on the exposed pixels.
There is therefore a need for a wireless X-ray imaging sensor panel of the type using amorphous or crystalline silicon photodiodes or metal insulated semiconductor (MIS) sensors that is capable of operating independently of the main imaging system and that can remotely and reliably detect both the onset and cessation of impinging X-rays from an X-ray source in the main imaging system.